A major flaw with lethality
You can easily counter it. Any AD assassin you face, since lethality early is really weak, as a mage, just build one armor/tank item after your first. Example, if you play akali, go gunblade into sunfire and the AD assassin can never kill you. This is because lethality does a %of armor based on the enemies level. So if they are even level, you get hardly any bonus, and the raw % is way too low to kill through the armor. This is a massive flaw I just found out and it can be abused.